1. Field of the Invention
This invention is in the field of disc brakes, and more particularly relates to compositions which can be applied to metal surfaces of disc brakes to eliminate brake squeak.
2. Description of the Prior Art
A conventional disc brake assembly is formed from a brake support, known as a yoke or caliper, disposed along the periphery of a rotor which is connected to the axle or wheel of a motor vehicle. The brake support is fixed to a non-rotatable part of the vehicle such as the chassis or axle housing. An actuating member, generally a hydraulically shiftable piston, is axially movable perpendicular to the braking faces of the rotor to urge a brake shoe or friction pad against the rotor which causes braking. Often, a pair of friction pads are provided in juxtaposition with one another on opposite sides of the rotor so as to sandwich the rotor between them. By providing a yoke or rotor which is movable axially, or by employing a pair of hydraulically driven pistons, provision is made to drive both friction pads into contact with the rotor.
Disc brakes have received increased acceptance among car manufacturers. One problem, however, is that disc brakes tend to squeak or squeal, even when they are not applied. This squeaking has persisted despite great efforts to eliminate it in the manufacture of brakes.
Many of the attempts to solve the squeaking of disc brakes have resulted in various mechanical devices intended to be mounted directly upon the brakes. A sampling of patented mechanical devices for such use includes the following.
U.S. Pat. No. 3,429,405 describes a generally plate-like or flat body of a vibration-damping material which is interposed between a brake shoe member and piston or brake support. This body is preferably formed from a synthetic elastomer or synthetic resin, constitutes one element of a self-adjusting or remotely controllable auxiliary mechanism operating with wedge surfaces and having a wedge body displacable in the direction of convergence of the wedging surfaces by manually operable or automatic means. The wedge body is constituted of elastically compressible material which may contain reinforcing materials. Examples of such materials include neoprenes, polytetrafluoroethylene, polyurethanes and superpolyamides, advantageously reinforced with steel layers such as steel cables or woven networks.
Similarly, in U.S. Pat. No. 3,720,293, a device for eliminating brake squeal is described which is formed by interposing a supporting member made of elastic material between the brake piston and piston cylinder in such a manner which allows the piston to slide freely inside the cylinder. This is claimed to eliminate vibration of the piston during braking because the piston and piston cylinder dynamically act as one body.
A heat-resistant damping element for insertion between the brake piston and friction pad or friction pad and brake mounting is disclosed in U.S. Pat. No. 3,885,651. The proposed damping element has plastic and elastic properties, i.e., visco-elastic properties similar to those of butyl rubber, and has a covering element adjacent to the damping element with elastic properties similar to those of a metal such as steel.
U.S. Pat. No. 3,918,555 describes a strip of substantially incompressible material coated on both faces with adhesive and adapted to be interposed between a friction pad assembly and a brake support. A suitable incompressible material is stated to be a strip of aluminum.
Still another mechanical device for eliminating squealing is described in U.S. Pat. No. 3,966,026. In this patent, a noise-suppression support for the friction lining of a braking member is described which consists of a plurality of thin metallic sheets or strips contiguous to each other over their whole surface and all assembled together by gluing, welding in zones, riveting insetting in a rim or flange, etc. Many of these mechanical devices require that the basic structure of disc brakes be significantly modified which would require expensive modifications to tooling, etc. Additionally, it is not certain that such modifications would not deleteriously affect the efficiency of disc brakes. Also, such changes, even if made, would not aid in eliminating squeak from the millions of disc brakes already produced and sold.
In addition to mechanical devices, such as those described above, it has also been proposed in U.S. Pat. No. 3,455,416 to introduce a high viscosity silicone fluid, so-called "bouncing putty," between the movable part of a brake assembly and a fixed part to provide a resilient cushion therebetween. Such materials are relatively expensive and probably would be expected to wear off the coated parts in a short time.
It has also been proposed to apply polymer coatings to the metal surfaces in disc brakes. One material suggested, for example, is polyvinyl acetate plasticized with dioctyl phthalate. This particular polymer composition, however, as well as most polymer compositions, does not have the combination of unique properties required for this application. Under the tremendous temperatures and pressures generated in disc brakes, polyvinyl acetate will melt, flow and lose essentially all of its plasticizer due to evaporation.
Despite the many efforts aimed at overcoming the squeak problem associated with disc brakes, the success has been very limited. There is simply not an inexpensive yet efficacious way for the owner of an automobile, for example, to eliminate squeaking or squealing from his disc brakes.